Biogeochemical and micro-facial fingerprints of ecosystem response to rapid Late Glacial climatic changes in varved sediments of Lake Meerfelder Maar (Germany)

The response of a lacustrine ecosystem to climatic changes from 13,500 to 10,800 BP was studied in a varve dated sediment profile of Lake Meerfelder Maar, western Germany. Bulk biogeochemical parameters, stable carbon and nitrogen isotopes of sedimentary organic matter and micro-facies analysis are used for a detailed investigation of the lake's development from the Allerod interstadial (AL) to the Younger Dryas stadial (YD) and into the early Holocene (Preboreal). Varve micro-facies reveal rapid changes in composition and seasonal structure of the depositional environment mainly in temporal accordance with changes of terrestrial biozones. The respective responses of bulk proxy parameters (similar tobi-decadal resolution) indicate different sensitivities to diverse climatic changes. A prominent transition took place within two decades at the AL/YD boundary (12,690-12,670 BP). Increased flux of nutrients released from redeposited littoral sediments and shorter YD summer seasons led to an acceleration and concentration of lacustrine primary production with reduced discrimination against (1 3) C. High lacustrine primary production was further favored by relatively warm YD summer temperatures. At 12,240 varve years BP, from 1 year to the next, a regular deposition of detrital layers in spring set in which is related to local hydrologic threshold processes amplifying, the response to increased snowmelt discharge. This distinct change in the middle of the YD is clearly detected by varve micro-facies but not equally recorded in organic bulk proxy data. Especially, carbon isotope ratios remain constant and indicate a negligible effect of this process on the productivity of lacustrine algae. The YD/Preboreal transition (11,645-11,585 BP) is marked by a characteristic change in micro-facies. Throughout this transition, stable carbon isotope ratios strongly decline while the accumulation of opal and organic matter (OM) remained constant. This is seen as an increasing importance of spring and autumn for gross primary production at the onset of the Holocene. (C) 2004 Elsevier B.V. All rights reserved